Fault is there and seems to be the work of Amaryllis worm. Attached pictures of the worm taken from below url.
viewtopic.php?f=96&t=1884&start=20
This was also pinpointed by Mr. Heinie in quotes below.
"The Amaryllis worm is the number one destroyer of any plant from the Amaryllidaceae family. They get into the top and bottom skins of the plant and eat very fast towards the crown of the plant. Once they get into the crown the plant had it and you can throw it away. When you see them early you can squash them in the leaf between your thumb and forefinger. The eggs are laid on the underside of the leaf. The egg is small light green and looks like a small droplet."
viewtopic.php?f=102&t=2660&p=24766&hili ... orm#p24766
More in quotes by Mr. Heinie.
"The Amaryllis worm is very active here during Spring and Summer therefore it means from September through to February. They are very happy to destroy the softer leaf plants like Clivia, Crinum, Agapanthus, Arum Lilies and Hippeastrum.
This is another worm we have to be on the lookout for here in the same warm months. The Hawk Moth also lay the eggs on the underside of the leaves at night and a tiny half millimeter worm grows to a 20mm thick worm in a matter of 2 to 3 days. They destroy a plant even faster than the Amaryllis worm. See the Arum Lily plant destroyed in two days by three worms. I squished the third one befor the photos."
viewtopic.php?f=102&t=2660&hilit=amaryl ... m&start=10